Small gas turbine engines have been built in numerous configurations. For example, some engines have a centrifugal compressor in a back-to-back arrangement with a radial turbine. These engines are poor candidates for applications having strict size limitations, because they generally have large diameters relative to their inlet air flow and hence, generate low thrust per unit of frontal area. Other small gas turbine engines consist of an axial compressor in axial arrangement with an axial turbine. These engines tend to have multistage compressors and turbines making them longer and mechanically more complicated than the centrifugal engines.
Holzapfel et al., U.S. Pat. No. 3,756,019 describes yet another gas turbine configuration in which a multistage turbine circumscribes a multistage compressor. The compressor and turbine are divided into two sections with each section having its own shaft. One disadvantage with the Holzapfel configuration is that the turbine blades have higher wheel speeds, (i.e. higher radius), than the compressor blades. Typically, the compressor in a gas turbine engine needs higher wheel speeds or more stages than its driving turbine in order to absorb the turbine's work. Thus, an engine configuration in which the turbine has higher wheel speeds than the compressor will require more compressor stages making the overall engine larger and perhaps requiring a multiplicity of shafts. Also, the Holzapfel configuration places the hotter turbine flowpath outside the cooler compressor flowpath. As a result, the engine's mount, and the surrounding airframe structure, referred to as the nacelle, must be made strong enough to withstand these hot temperatures, resulting in heavier components.
Accordingly, a need exists for a small, light weight gas turbine engines that overcomes the disadvantages found in the prior art, small gas turbine engines.